/Engine/Source/Runtime/NullDrv/Public/NullRHI.h
Watch out for OutData to be 0 (can happen on DXGI_ERROR_DEVICE_REMOVED), don't call RHIUnmapStagingSurface in that case.
virtual void RHIMapStagingSurface ( [FRHITexture](API\Runtime\RHI\FRHITexture) * Texture, [FRHIGPUFence](API\Runtime\RHI\FRHIGPUFence) * Fence, void *& OutData, int32 & OutWidth, int32 & OutHeight, uint32 GPUIndex )